Ubiquitin is a small (76 residue) protein containing a single helix and a mixed (parallel + antiparallel) ?-sheet surrounding a single hydrophobic core. It has no disulfides, cofactors or cis peptide bonds that might complicate folding. We have designed a single tryptophan mutant (F45W) which can serve as a probe of folding/unfolding. Comparing fluorescence and IR spectra will help to reveal intricacies of a known folding intermediate. Preliminary measurements of fluorescence lifetimes and intensities in both folded and unfolded species have been determined.